It is known to manufacture a vehicle frame by providing separate subassemblies, each subassembly being composed of several separate components. Each subassembly is manufactured by joining together several tube-type members with tube and socket joint or by means of a node structure that can be a cast component. A cast node can be formed from a single cast member or two or more cast members that are joined to form a node. A node typically consists of a structure with one or more connection points, e.g., arms or sockets, to which, for example, a cast, extruded, or sheet member can be connected by various joining techniques, such as by means of weld, adhesives, or mechanical devices. The frames and subframes can be assembled by adhesive or other bonding or by combinations of these and other joining techniques. An example of such a vehicle frame structure is available in U.S. Pat. No. 4,618,163, entitled "Automotive Chassis" the contents of which are incorporated herein by reference. This structural configuration for a vehicle frame is often referred to as a "space frame." Aluminum is a highly desirable metal for such vehicle frame constructions because of its light weight compared to a typical steel component and aluminum's energy absorption properties. Aluminum alloys also improve the vehicle's frame stiffness. More importantly, an aluminum vehicle frame demonstrates the strength and crash worthiness typically associated with much heavier, conventional steel frame vehicle designs. The lightweight aluminum vehicle frame also provides numerous environmental benefits and efficiencies through reduced fuel consumption and the opportunity ultimately to recycle the aluminum frame when the useful life of the vehicle is spent.
Conventional aluminum/magnesium casting alloys have many attractive properties, such as high ultimate tensile strength (&gt;40 ksi) and elongation (&gt;8%) with moderate yield strength (&gt;16 ksi). However, in the preparation of component parts for automotive frame assemblies, sub assemblies, and components, it is desirable to have component parts characterized by higher elongations, while maintaining acceptable strength, stress-corrosion resistance, and other properties important to vehicle "space frame" applications. Prior to the instant invention, existing aluminum/magnesium alloys failed to exhibit the desired property requirements.
Current practice in the manufacture of automotive components used in a "space frame" structure as disclosed in the aforementioned U.S. Pat. No. 4,618,163 includes using aluminum/silicon casting alloys. One example of such an aluminum/silicon casting alloy is disclosed in U.S. Pat. No. 5,250,125, entitled "Process for Grain Refinement of Aluminum Casting Alloys, in Particular Aluminum/Silicon Casting Alloys" to Koch et al., the contents of which are incorporated herein by reference as if fully set forth. By way of an additional example, the assignee of the instant invention has previously disclosed in U.S. Pat. No. 5,076,344 entitled "Die Casting Process and Equipment," a casting alloy capable of meeting the requirements of the space frame cast nodes without the economic liability of expensive constituents. The use of aluminum/silicon alloys requires the post casting solution heat treatment, quenching, and aging of the cast component in order for the component to exhibit the desired mechanical properties. Unfortunately, solution heat treatment and quenching can often cause some degree of distortion to the cast component and the reworking of the cast component to correct heat treatment distortion is a time and labor intensive activity. Notwithstanding such efforts to develop alloys that offer the desired properties and characteristics, there remains a need for alloys that are cost effective and that are much less sensitive to heat treatment subsequent to the casting operation while meeting all of the property requirements described above.
It is an object of this invention to provide an aluminum/magnesium alloy ideally suited for use in die casting operations.
It is another object of the invention to provide an aluminum/magnesium alloy product characterized by the elimination of solution heat treatment.
It is also an object of this invention to provide an aluminum/magnesium alloy capable of an increased range of shapes and improved dimensional stability for use in the manufacture of aluminum intensive vehicles.
It is an object of this invention to provide improved cast products and components consisting of an improved aluminum/magnesium alloy cast members that ideally are suited for frames, subframes, and frame members in vehicle primary structures.